Be sure to evaluate motive, means and opportunity for each suspect before filing for your arrest warrant:
Who had a reason to kill the victim? Did any particular person benefit—financially or romantically—from the victim's death? Or was it a case of revenge?
Who had access to the murder weapon and victim? Did any particular person have the unique knowledge, skill or physical ability necessary to use the murder weapon?
Who had a chance to kill the victim? Is any particular person unaccounted for (has no alibi) during the time the victim was killed?
Try these if you are still stuck
Who did it?